The present invention relates to a system for precise, controlled movement of a usually planar object, such as a photographic or holographic transparency, and is especially useful in any situation requiring rapid random access to a position in a plane. Examples of such situations include photocomposition, microfiche data retrieval, digitally controlled plotting, computer generation of images, digitally controlled manufacture of electronic circuits and machine parts, and analysis of specimen samples supported in a planar device. As is apparent, several of these situations are direct to obtaining random access to information stored as an image in the plane.
In general, prior methods for obtaining such random access fall into two distinct categories: stroboscopic devices and stepping devices.
In stroboscopic devices, the plane is formed into a disk, drum or cylindrical belt and is rotated continuously. Access to the information at a specific locale is obtained by flashing one of several properly located stationary light sources at the proper instant of time to project or read the desired image while it is in motion.
In stepping devices, the plane is moved to the desired position by electro-mechanical driving devices, typically stepping motors.
Neither of these prior methods is fast enough for many present applications. The speed of stroboscopic devices is limited by image blur and available light. Moreover, continuous motion is not acceptable for many random access applications. The speed of stepping devices is limited by the high mechanical and electrical inertia associated with an iron armature and field pole structure. The driving force available is limited by magnetic saturation. Stepping motors also have poor transient response: they are unable to follow high stepping rates from the start, the they have long settling-down time when stopped. Moreover, the uncertainty in stopping position represents a sizeable fraction of a single step, thus requiring a large number of steps to be assigned to each unit space in order to achieve the required position accuracy. Even sophisticated efforts to improve this situation (by use of starting ramps and feedback control) have failed to adequately improve the transient response of stepping motors.